题目内容
15．People under 21are not to drink in the USA．（ ）| A． | allow | B． | allowed | C． | allowing | D． | allows |
分析 在美国，21岁以下的人不允许喝酒．
解答 答案：B．
首先根据语境推测句意是"在美国，21岁以下的人不允许喝酒．"，由于句子表述的是一般性的事实，所以用一般现在时，主语People under 21 是动作allow 的承受者，所以用被动语态，一般现在时被动语态的构成是：主语+am/is/are+动词的过去分词，变否定句在be后面加not，故选B．
点评 本题考查时态和语态．每个时态都有标志词，熟记这些标志词是做题的关键．在没有时间状语的情况下可以根据语境来对时态做出判断．判断语态的关键则是观察主语是谓语动词的执行者还是承受者．
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